Asphalt pavement is one of the most recycled materials in the world, finding uses when recycled in shoulders of paved surfaces and bridge abutments, as a gravel substitute on unpaved roads, and as a replacement for virgin aggregate and binder in new asphalt pavement. Typically, use of recycled asphalt pavement is limited to sub-surface pavement layers or to controlled amounts in asphalt base and surface layers. Such uses are limited in part because asphalt deteriorates with time, loses its flexibility, becomes oxidized and brittle, and tends to crack, particularly under stress or at low temperatures. These effects are primarily due to aging of the organic components of the asphalt, e.g., the bitumen-containing binder, particularly upon exposure to weather. The aged binder is also highly viscous. Consequently, reclaimed asphalt pavement has different properties than virgin asphalt and must be processed in such fashion that the properties of the aged binder don't impact long term performance.
To reduce or retard the impact of asphalt aging on the long-range performance of mixtures, numerous materials have been investigated. For example, rejuvenators are marketed with a stated goal of reversing the aging that has taken place in recycled raw materials such as reclaimed asphalt pavement (RAP) and/or reclaimed asphalt shingles (RAS). It is unlikely that rejuvenation of asphalt can actually occur and the more likely scenario is that these additives may instead serve as softening agents for the virgin binders employed in mixtures containing RAP and/or RAS. In some instances, 10% or more by weight of these softening agents are added to the virgin binder when such mixtures are produced.
Aging can be assessed by measuring ΔTc, the difference between the Stiffness critical temperature and the creep critical temperature after aging. The use of softening agents can produce a mixture with recovered binder properties that have acceptable values of ΔTc after extended mixture aging, but these acceptable binder properties after aging come at the cost of producing a mix that can be quite low in stiffness during the pavement's early life.